1. Field of the Invention
This invention relates generally to the field of seismic surveying and more particularly to a no-liquid filled streamer cable for use in performing seismic surveys under water.
2. Description of the Related Art
To perform seismic surveys over water-covered areas, such as offshore, to obtain information about subsurface formations for recovery of hydrocarbons (oil and gas), one or more strings of hydrophones are towed behind a vessel designed for performing seismic surveys. Such strings of hydrophones are typically referred to in the art as "streamer cables" or "towed arrays." For three dimensional seismic surveys, several streamer cables (generally between 4 and 12) are deployed simultaneously, each such cable extending usually between three (3) and eight (8) kilometers. Each streamer cable is normally made by serially joining smaller sections of 75 meters to 150 meters in length, referred to in the art as "active cable sections." The streamer cables are generally towed twelve to thirty feet below the water surface to reduce the effect of surface waves and surface reflection noise on the hydrophones. However, towing streamer cables at such depths requires great pulling force, hence the need for larger and more powerful vessels, which in turn increases the capital and operating costs. It is, therefore desirable to have streamer cables which are less susceptible to surface waves so that they may be deployed near the water surface.
To perform a seismic survey, acoustic shock waves are generated at selected points in relation to the streamer cables. These shock waves travel down to the subsurface formations and are reflected by subterranean bed boundaries back to the streamer cables through the earth and water. The reflected shock waves are detected by the hydrophones, which produce corresponding signals. These signals are processed to obtain seismographs of the earth's subsurface beneath the streamer cables.
Each active cable section of commercially available streamer cables is typically made up of a flexible sealed tubular outer jacket made from polyurethane or a similar material. Multiple (generally between two and five) strain members in the form of cables made from steel or aramid fibers or other high strength materials, such as those sold under the trade names of Kevlar or Vectran, are spaced apart axially along the entire length of the active cable section. Typically, the strain members are deployed near the inside surface of the flexible tubular member to absorb the pulling forces when the streamer cable is towed behind the vessel. The hydrophones are typically placed in the space between the strain members. To detect very small reflections from the subterranean formations, equispaced groups of hydrophones (typically between 8 and 14) are placed in each active section. A one hundred (100) meter active section typically may use between 96 and 150 hydrophones. Electronic circuitry, such as preamplifiers, circuits to digitize the analog hydrophone signals and wires to provide two way signal and data communication between the active section and control units located at the vessel are placed between the active sections. Since the streamer cable is towed at a predetermined depth below the water surface, the cable is made to have a desired predetermined buoyancy. For this purpose, all of the empty space inside the outer housing is filled with a nonconductive light fluid, such as kerosene.
The above-described streamer cables suffer from a number of significant problems. The outer jacket is typically only a few millimeters thick and is, thus, relatively weak. The streamer cables are normally spooled on large drums for storage on the vessels and often rupture during winding (spooling) and unwinding operations. Additionally, the outer jacket often ruptures during towing, due to fishing boats inadvertently passing over the streamer cables, fish bites and the streamer cable becoming entangled with offshore structures. Seismic survey companies spend large amounts of monies in repairing such cables and typically keep excessive inventory of such cables. Outer jacket ruptures during surveying operations can require shut down of the surveying operations. Such down times can be very expensive due to large capital cost of the vessels and the lost time of the crew, which can be several thousand dollars per hour.
The fluid in the cable also causes a number of problems. As the streamer cable is towed, waves are created within the fluid in each active section. Such waves tend to impart noise in the hydrophones, thereby degrading the quality of the detected signals. Further, each time a surface wave crashes into the streamer cable, it creates an acoustic source, which transmits through the fluid in the form of noise. Additionally, fluid-filled streamer cables are affected by the ripples at the water surface. For these reasons, fluid-filled streamer cables are towed several feet, usually between 12 and 30 feet, below the water surface. Additionally, kerosene is toxic and highly flammable, which creates safety, health and environmental problems. Any streamer cable fluid leaking into the ocean is also hazardous to marine life. Attempts have been made to design streamer cables that are not filled with any fluid and do not utilize flexible outer housings or shells. However, such streamer cables have not been commercially successful. The present invention addresses these problems and provides a streamer cable, wherein a foam is extruded during the manufacturing of the cable to provide the desired buoyancy. The outside of the cable is made from an extruded jacket of a material that is sufficiently strong to provide a relatively rugged surface for handling. The streamer cable of the present invention is not susceptible to ruptures as are the fluid-filled streamer cables.
The use of multiple stress members causes several problems. First, it reduces the useful space available in the active section for the placement of hydrophones and desired electronic circuits. Second, the strain members are typically not identical, which causes uneven load sharing between the various strain members. Third, the strain members outside the hydrophones vibrate under load and thereby create noise in the hydrophones. These strain members are subject to failure because of bending loads during reeling. The present invention addresses these problems and provides a steamer cable that utilizes a single central strain member. Hydrophones are placed on the outside and around the stress member. The single strain member in the middle is lighter than the combined weight of multiple stress members of the prior art streamer cables, allows forming hydrophones around the strain member instead of in the space between the multiple stress members, and is less affected by bending loads during reeling. This design allows building a relatively small diameter and lighter cable compared to the commercially available fluid-filled streamer cables.
As noted earlier, typical prior art streamer cables utilize between 96 and 150 hydrophones per 100 meter section. Groups of hydrophones are usually used to detect signals corresponding to a single point in the cable. Signals from all the hydrophones in each group are combined to increase the signal amplitude and to reduce the effect of various types of noises present in the fluid-filled streamer cables. A number of hydrophones are utilized partially to compensate for such noises. The cable design of the present invention is inherently less noisy and, thus, allows for the use of relatively fewer number of hydrophones per active cable section.
Additionally, the present invention provides a hydrophone that is placed around the stress member that has higher signals to noise ratio compared to the prior art hydrophones and receives unimpeded signals from the subterranean formations surfaces, i.e., around 360 degrees around the streamer cable. Due to the availability of space between the inside of the hydrophone and the central strain member, some of the essential electronic circuits, such as preamplifiers, can be placed within the hydrophone, thereby reducing the distance between the sensor and the preamplifier.